1. Field of the Invention
The present invention relates to an image forming apparatus which uses an electrophotographic method, and more particularly, to an image forming apparatus, such as an electrophotographic printer, a facsimile apparatus, or the like, which uses a laser beam or a light-emitting diode (LED) array as a light source.
2. Description of the Related Art
One type of the above-described image-forming apparatus is divided into an upper-case portion and a lower-case portion. As shown in FIG. 4, an upper case B is openable and closable relative to a lower case A around a hinge 30. The lower case A contains a sheet feed means a, an image transfer means b, a plurality of transfer-material conveying means c, a driving means (not shown), a fixing means d, and the like. The upper case B contains a cartridge guiding/holding means e for retaining an image-forming process cartridge 7, an optical system means (an exposing light source means f, such as a laser-beam scanner or the like), a sheet discharge means g, and the like. By opening the upper case B around the hinge 30, the acts of mounting and demounting the process cartridge 7, removing jammed transfer material, and the like are performed.
However, such an apparatus has a disadvantage because the upper and lower cases are independently configured. Consequently, it is difficult to provide positional accuracy and alignment, i.e., parallelism among the several components and units. Particularly, in a transfer portion, the positional relationship between the images on a recording material and a photosensitive member is often skewed, deviated, or tilted. It is also difficult to provide positional accuracy between a driving member within the process cartridge for receiving the driving source of the photosensitive member and the gears of the driving means in the lower case. Hence, the center distance between the gears deviates, causing unevenness in the rotation speed of the photosensitive member. As a result, unevenness in pitch, i.e., unevenness in the distance between adjacent scanning lines on the photosensitive member occurs in the image. In an apparatus wherein the frame of the main body of the apparatus is configured by facing side plates made of sheet metal which are connected by individual metal stays, the side plates may become twisted or tilted with respect to each other. Accordingly, it is difficult to increase the dimensional stability of the frame. Moreover, a frame made of sheet metal often resonates due to vibration caused by the driving system, causing an inferior image, such as a blurred image, or the like.